The most common causes of such prolonged rashes is a continued Allergy/Atopic dermatitis. This is often provoked by continued and repeated exposure to some allergen. You need to pay careful attention to your environment to see if you can find out any allergen which could be the cause.
Insect bites are a possibility. Mosquite, horsefly, ticks, midge or gnats, fireants, chiggers, etc can cause such reactions. It could also be related to certain medicines, foods, suppments, etc. Check out whether there is any medication associated with the rash. . Also try to recall any associated foods, specially foods like nuts, some new types of meat, fish, etc before this rash started. You could be having a food allergy. You should also watch out for any soaps or detergents. Cosmetics like moisturisers could be the cause. Also try to recall any activities associated with this, like hiking to some place, cleaning the house or some area of the house, some hobby like wood working etc which coincide with the rashes.
If you find anything which could have caused the allergy, then you have to avoid it. However, quite as often, no definite cause of such allergic reactions can be found, do not worry if this is so. Most of the time, cause for Atopic dermatitis is not detected.
You should take an antihistaminic like Cap. Claritin (OTC) one capsule daily. Also apply 1% Hydrocortisone ointment. This will get rid of the rash.
However, if the rashes keep recurring, you should see a doctor. Other causes like scabies and fungal infections may be implicated. Less common causes like skin infections, autoimmune conditions like psoriasis, etc would need to be ruled out.
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